I hadn't had to learn a word: somehow my association with the Doctor had enabled me to understand any language I came across.Bernice Summerfield [src]

During the incident with the Sycorax on Christmas2006, the translation circuit worked for all humans on the Sycorax ship, even though the Tenth Doctor was not, at the time, apparently aware of their presence. (TV: The Christmas Invasion) Even when Davros had swapped minds with the Doctor, he was able to 'share' his body's gift of translation with Flip Jackson and Jared Ramon when they arrived back in 1815, allowing them to understand French despite the fact that Flip and Jared had never been inside the TARDIS (AUDIO: The Curse of Davros). On yet another occasion, the Seventh Doctor told Elizabeth Klein that the telepathic field was limited to a certain radius around the TARDIS, with the Doctor losing the ability to communicate with the native Vrill once he moved beyond that radius, although this can be attributed to the fact that the Vrill communicated via smell, with the result that the language was so complex that the Doctor would be unable to even attempt to speak it on his own. (AUDIO: Survival of the Fittest)

The Ninth Doctor once mentioned that the translation systems had a swear filter that prevented the passengers from hearing any swear words; even an angry cavewoman's rants were translated as her saying, "Blinking", rather than a more apt phrase. (PROSE: Only Human)

The translation circuit was apparently capable of translating for creatures without a language. Marnal, the previous owner of the Doctor's TARDIS, identified the animal language translators as one of the many functions in the TARDIS that was no longer in operation after the Doctor's botched repair work on his ship, but the Eighth Doctor dismissed these as irrelevant (PROSE: The Gallifrey Chronicles). However, it would appear that the Doctor would later at least attempt to repair these circuits, as the Eleventh Doctor was able to barely communicate with a Krafayis, which were only of animal-level intelligence (TV: Vincent and the Doctor), and later apparently understood a horse when it told him its name (TV: A Town Called Mercy).

Multilingual individuals were sometimes capable of identifying what language was being spoken. When the Fourth Doctor spent time conversing with Emmeline Neuberger, a native German speaker who also spoke reasonably good English, he responded to her with equal ease when she spoke to him in English and German without showing any sign that he noticed her change in language, Emmeline noting that she couldn't tell what language he was addressing her in. (PROSE: Wolfsbane) The Doctor tended to have more luck in identification. The Fourth Doctor identified the mercenary trader Garron as originating from Somerset simply by hearing his accent, despite the fact that he was on the planet Ribos at a time before it had become aware of other worlds and hence would have no reason for an Earth native to visit it. (TV: The Ribos Operation) On another occasion, the Tenth Doctor used Madame de Pompadour's particular French accent to identify the century in which she lived, while Mickey simply assumed she was speaking English. (TV: The Girl in the Fireplace) When Rose was talking to a pair of African mercenaries, she continually heard them talking in English even when they explicitly said that they would switch to an obscure African dialect that was only known in a small part of the continent in the twenty-second century (PROSE: The Art of Destruction).

The TARDIS translations were closer to the original language than other translators, although it would allow 'local' translations to take over if languages were already being translated. Selachianbattlesuits had built in translators, giving them a harsh voice. When removed from their armour and in range of the TARDIS, their speech sounded more melodic, closer to the Ockoran's natural song-like language. (PROSE: The Final Sanction) Likewise, the translation circuit did not compensate for the glitch in Sil's translation system that caused his voice to sound so disturbing (TV: Vengeance on Varos).

It has been suggested on some occasions that the TARDIS translation also slightly modified peoples' minds so that they did not consciously register that they should be unable to understand the languages they were listening to. The Doctor was able to deduce that Sarah had been hypnotised when she directly asked how she was able to understand Italian during a trip to the fifteenth century. (TV: The Masque of Mandragora) On another occasion, when the Brigadier asked how he was able to understand German during a trip to a party thrown by the Nazis in Hitler's honour in 1942, the Sixth Doctor reflected that the champagne the Brigadier had drunk was probably responsible for him asking the question. (PROSE: The Shadow in the Glass)

As noted by Bill Potts, the circuit also appears to handle lip synching, in that the speaker being translated will also appear to be moving their lips in time with what is being said in translation. (TV: The Eaters of Light)

When the Fifth Doctor and Vislor Turlough initially encountered the Phiadoran/VrallLytalia on the moon in 1878, Turlough assumed that his inability to understand her was natural, but the Doctor soon realised that they had only failed to understand her because her language syntax was being replaced by English based on knowledge extracted from the brain enzymes of Sub-Lieutenant Granby, and she had merely been making random sounds to conceal this until she had another excuse to speak English (PROSE: Imperial Moon).

The Hervoken language, encountered by Martha and the Tenth Doctor, couldn't be fully translated, though the essence was understood. (PROSE: Forever Autumn)

Some languages that were too complex, like the Sittuun language encountered by Amy and the Eleventh Doctor, would not be translated. (PROSE: Night of the Humans) In other instances, if the idea was too complex, translation would be incomplete or faulty, an example being when the Fifth Doctor used the phrase, "We will have been here before", to describe the TARDIS arriving at a crater on the Moon in the early twenty-first century when the TARDIS would visit that location in 1878 in its personal future. (PROSE: Imperial Moon) The Seventh Doctor and Mel encountered Golosian, which was also too complex to be translated by the TARDIS, although a translator created during their visit to Dark Space 8 was able to provide an accurate translation of the Golosian language. (AUDIO: Bang-Bang-a-Boom!) The Foamasi language also went untranslated. (TV: The Leisure Hive) This was apparently due to the complexity, as the language stimulated the visual cortex of the brain, meaning Foamasi effectively saw their language. (PROSE: Sleepy) On the other hand, Judoonese was too basic to be translated. (AUDIO: Judoon in Chains)

On some occasions, the TARDIS translation circuits could fail to work through deliberate action on the part of others. On one occasion, the Master disrupted the Third Doctor's attempt to talk to him by hacking the translation and telepathic circuits of the Doctor's TARDIS and feeding the Doctor's words back to him before he had spoken, resulting in his words coming out backwards. (TV: The Time Monster)

When facing the alien artist Monos, who sought to use a psychic gauntlet to imprint his identity across London, the Doctor was able to defeat him by having Amy and Rory deliberately sabotage the translation circuit, scrambling all language in the ship's vicinity so that nobody could process Monos's name properly, giving the Doctor time to disable his equipment (COMIC: Sticks and Stones).

Most of the Doctor's companions were native speakers of English, and the Doctor himself was a "master of English" — indeed a fan of the language. (AUDIO: ...ish) However, he did occasionally travel with non-English-speakers. Elizabeth Klein, a native German speaker, required the circuit just to communicate with the Doctor. (AUDIO: Survival of the Fittest)

The circuit could apparently be influenced by the passengers who travelled in it. When the Eighth Doctor, Fitz and Anji landed on a world where they encountered the "mooncalfs" - people born with some kind of genetic deformity, regarded as an abomination in this world - Fitz recalled the term from his childhood as referring to someone who was either a bit slow or someone considered a freak, speculating that the TARDIS translator chose a word that he knew to describe them because it was more "tuned in" to his "wavelength" given that he had spent longer travelling with the Doctor than Anji. (PROSE: Vanishing Point)

Anji had a particularly unusual relationship with the translation circuit. When dealing with the sentient tigers of the planet Hitchemus, she was actually able to ignore the circuit's attempts to translate at first because she initially couldn't accept that the tigers were sentient. (PROSE: The Year of Intelligent Tigers) She actually learned a new language whilst travelling with the Doctor. Caught in the midst of the Spanish Civil War, she used her pre-existing fluency in French to quickly assimilate Catalan while she was in Barcelona. (PROSE: History 101) It was unknown how she was able to experience traditional language learning whilst still nominally linked to the TARDIS.

When travelling with Jago and Litefoot, Litefoot immediately noticed that the Venusians were speaking English when they should be speaking an alien language. Jago, on the other hand, thought Venusians would naturally speak English. (AUDIO: Voyage to Venus)

When Troy Game first travelled in Lord Roche's TARDIS, as she was already naturally capable of speaking other languages due to her race's innate telepathy, the TARDIS instead augmented her natural abilities, increasing her telepathy and granting her a degree of empathy. (PROSE: The Suns of Caresh)

The TARDIS crew's ability to understand other languages wasn't explained in any story in any medium until The Masque of Mandragora. Even so, that was the only story in the whole of the 1963 version of Doctor Who to even touch upon the subject of language translation. The issue has had much greater prominence in the BBC Wales programme, which has then had an impact on the Big Finish past Doctor stories.